The present invention relates generally to a process of forming patterned structures on a substrate utilizing a bilayer metal lift-off technique and, more particularly, to the use of azo dyes as an additive to polydimethylglutarimide (PMGI) to improve adhesion of the PMGI layer applied to an underlaying substrate.
The use of a hi-layer resist lift-off process in the fabrication of integrated circuit components and other thin film structures such as field effect transistors (FET), conductor patterns and magnetic sensing transducers, for example, is well know in the art. For example, U.S. Pat. No. 4,814,258 granted to Tam discloses a bi-layer lift-off process utilized for the fabrication of various types of FETs, and European Patent Application No. 0 341 843 published Nov. 15, 1989 discloses a bi-layer metal lift-off process for forming conductor patterns on a substrate.
Basically, the bi-layer lift-off system comprises a release layer formed on a suitable substrate which is then covered by a top imaging layer of photoresist. A Diazonapthoquinone (DNQ)/Novolac positive resist is suitable for use as the top imaging layer. Polydimethylglutarimide (PMGI), a polymer supplied by the Shipley Company, is a suitable material for the release layer. The top imaging layer is exposed and developed to provide the desired pattern. The release layer is then flood exposed and developed to expose the substrate surface for subsequent deposition of the desired structural features. During the development step, the release layer is undercut from the edges of the resist pattern a desired amount to facilitate the subsequent lift-off step.
A major difficulty and limitation of the bi-layer lift-off process utilizing PMGI as the release layer is the loss of or reduced adhesion of the PMGI layer to the underlaying substrate surface at lower prebake temperatures. Good adhesion of PMGI to various substrate materials has been obtained by oven baking at temperatures in the range of 190.degree. to 290.degree. degrees C., near or above the glass transition temperature for the PMGI resin. However, bake temperatures below 150.degree. degrees C. have resulted in, at best, marginal adhesion characteristics. The relatively high prebake temperatures required for suitable adhesion in PMGI systems can result in oxidation of the underlaying deposition surface, particularly certain metals, further resulting in reduced yields and degraded performance of the finished product.